1. Field of the Invention
This invention relates to an antenna device used in a base station antenna or the like for mobile communications, and more particularly to an antenna device in which a desired directivity angle is realized by a simple construction.
2. Description of the Related Art
In a base station antenna for mobile communications, a service area from which communication service can be provided is influenced remarkably by a directivity in the horizontal plane of the base station antenna. In case of establishing a base station antenna, it is desirable to locate it in the best place where all the service areas intended to cover can be fully provided without accompanying any unnecessary details. In other words, even if an electric power is delivered to an area where no mobile station exists, resulting in a loss of energy, while it becomes a problem, when no electric power is delivered to an area where some mobile stations exist.
In reality, there is a case where the best place is on a road or the other places where an antenna is hardly set up. Thus, there are frequently such a case where an antenna must be set up in a place where is that near to the best place, i.e. a next-best place. For instance, when it is intended to contain principally a service area along a longitudinal direction of a road 91 as shown in FIG. 1 by the use of an antenna having a directivity of an 8-figure shape in the horizontal plane, the best place is in the central point 92 of the road. However, the antenna cannot be located at the central point of the road, so that it is disposed on an electric pole, a telephone booth and the like positioned at a side of the road in reality.
In this case, however, when a base station antenna having an 8-figure shape directivity 94 is set up at a point 93 on a side of the road as shown in FIG. 1, an area contains inevitably a region which is not required principally for a service area, in other words, a building 96 faced to the road which is unnecessary for the service area is contained inevitably therein, so that there are useless regions. In FIG. 1, even if an antenna having omnidirectivity (a circular directivity) in the horizontal plane is used in place of an antenna having an 8-figure shape directivity, a demand for containing principally a region extending along the longitudinal direction of the road 91 cannot be attained.
A directivity in the horizontal plane of an ideal antenna suitable for a place shown in FIG. 1 in which an antenna is to be set up is that as indicated by a broken line 97. When the antenna has the directivity as indicated by the broken line 97, it can reduce a region in a service area covering the building 96.
In recent years, the number of mobile stations existing in a narrow area increases with progress of mobile station instruments. In this connection, when a base station antenna applying an omnidirectional antenna thereto is set up as shown in FIG. 2A to establish a circular service area surrounding a setting place 201, the sufficient number of channels cannot be ensured for the number of mobile stations, because of the limited number of channels which can be provided by a single base station. Under the circumstances, it is considered for ensuring the sufficient number of channels in each service area that a plurality of antennas each having a comparatively narrow directivity is set up at the same place, whereby different directions are covered to establish service areas, respectively. For instance, when three antennas each having 120° directivity angle are set up at the same setting place 201 as shown in FIG. 2B, service areas each having a sector form directing to a different direction, respectively, are shared, so that the number of channels being three times larger than that in the case where circular service areas are set up can be ensured.
However, when the number of mobile stations increases further, it is required that four or more of antennas are set up at the same setting place 201 so as to obtain narrower service areas as shown in FIG. 2C. In this case, since each service area 202 has each narrower angle, a directivity must be remarkably narrowed in each antenna.
Japanese patent application laid-open No. 11-31915 (prior art 1) discloses such a technology that omnidirectivity is obtained over a comparatively broad band by means of a substrate type antenna wherein electric supply lines composed of microstrip lines and antenna elements composed of microstrip lines are formed on a dielectric substrate.
On one hand, Japanese patent application laid-open No. 8-125435 (prior art 2) discloses such a technology that a reflector plate is opposed to an omnidirectional antenna, whereby such characteristics wherein a characteristic configuration of omnidirectivity is shifted unidirectionally are obtained, so that a circular service area is deviated away from a building.
However, even when a reflector plate is disposed so as to oppose to an omnidirectional antenna as in the prior art 2, only a directivity with a narrow directivity angle is obtained. Accordingly, such directivity in the horizontal plane of an antenna which can reduce a service area covering a side of the building as desired in FIG. 1, in other words, a wide directivity more than 120° directivity angle is not easily obtained. On the other hand, the directivity in the horizontal plane indicated by the broken line 97 cannot be obtained by an omnidirectional antenna as in the prior art 1.
Furthermore, when plural antennas are set up at the same place in order that a service area is divided into narrower regions, it is required that a directivity angle of each of the antennas has a desired narrow angle in response to the number of division.
As is apparent from the above description, such an antenna in which a desired directivity angle extending over a range of from a wide directivity angle of 180° to a narrow directivity angle of about 30° is obtained by a simple construction is demanded.